The invention relates to a method and apparatus for measurement of the diameter and runout, of an optical waveguide preform during manufacture of the preform. The runout is the deviation of the preform from cylindrical symmetry which includes deviation from roundness or straightness.
The manufacture of optical waveguides involves a series of carefully controlled steps. In addition to optical properties such as attenuation, dispersion, including polarization mode dispersion, numerical aperture and cut off wavelength, the fiber geometry must be controlled to small tolerances. All of these properties to a large extent are built into the waveguide fiber in the preform manufacturing step.
While feedback systems at the draw can generally maintain fiber diameter within specification, diameter control is facilitated by preform uniformity. Also, other fiber geometry derives directly from the geometry of the preform. Small deviations in preform geometry, of the order of a tenth of a millimeter, can produce out of tolerance waveguide fiber.
Typical waveguide geometry specifications include, outside diameter, core diameter, concentricity of core and clad and roundness of core and clad. Each of these quantities depends substantially upon the shape and symmetry of the waveguide preform. In addition, for singlemode waveguide fiber, cut off wavelength depends upon the size ratio of core and clad. A measurement of preform geometry is therefore essential for selecting those preforms which ultimately can be drawn into a waveguide fiber which meets the specified limits and tolerances.
However, because preform manufacture is typically done in a hot, corrosive environment, which may include a flame or other source of heat and light, attempts to measure preform geometry during manufacture have not been successful.
Further difficulties in preform measurement during manufacture are:
the measurement must not involve mechanical contact, because such contact with the preform surface can disturb the laydown process thereby upsetting the preform geometry or the preform drawing characteristics; PA1 the heat and light associated with the deposition process tend to blind or destroy non-contact sensors; PA1 the measurement should be continued for the entire manufacturing time, because defects can occur at any stage of the preform manufacturing process; and, PA1 the measurement must be accurate to at least a millimeter and preferably a few tenths of a millimeter. PA1 the preform surface must not be touched or disturbed; PA1 the measurement should be accurate enough to detect sub-millimeter deviations in preform geometry; PA1 the measurement apparatus must be stable under the severe environment of perform manufacture; and, PA1 the measurement must be essentially continuous for the duration of the preform manufacturing process. PA1 providing at least one distance gauge, spaced apart from the preform and having a sensing unit and a control unit, to produce an electrical signal proportional to the distance between the sensing head and the preform surface; PA1 dividing the electrical signal into a constant portion and an alternating portion; PA1 recording the signals and translating the respective constant and alternating signals into a measure of preform diameter and runout.
Thus, fatal preform defects typically are not found until the preform has been drawn into waveguide fiber and measured, i.e., after full manufacturing cost has been incurred.
Each step in the waveguide fiber manufacturing process consumes energy, costly liquids and gases, machine time and labor. The preform manufacturing step is the starting point of the process. Thus, identifying a defective preform during manufacturing results in substantial energy, raw material and labor cost savings and greatly improves utilization of factory equipment.
Hence there is a need is for a measurement during the preform manufacturing step which is predictive of the quality of waveguide fiber drawn from the preform.
The characteristics of such a measurement are: